(a) Field of the Invention
This invention relates to an improved process for synthesizing urea, and more specifically to a process for synthesizing urea which process features improved thermal economy.
(b) Description of the Prior Art
With the skyrocketing energy prices in recent years, thorough recovery of thermal energy has also been attempted in the production of urea so as to reutilize the thus-recovered thermal energy in the urea synthesis system and, when there is any surplus thermal energy left, to reclaim it as low-pressure steam. Steam consumption in each production process of urea is expressed in terms of a difference between the thus-reclaimed low-pressure steam and high-pressure steam which has been required for the separation of unreacted substances in the system. As the above difference becomes smaller, the production process of urea is considered to have been improved further.
The so-called stripping process is known as one of processes of the above-described type. In this particular process, unreacted ammonium carbamate and excess ammonia (hereinafter called generically "unreacted substances") in a urea synthesis melt are stripped by carbon dioxide or ammonia supplied as a raw material under the same pressure as the urea synthesis pressure while heating same with high-pressure steam of 20 kg/cm.sup.2 G or higher, thereby to decompose and separate the unreacted substances. A resulting gaseous mixture consisting of the thus-separated ammonia and carbon dioxide as well as carbon dioxide or ammonia which has been employed as a stripping gas for the decomposition and separation of the unreacted substances is condensed at substantially the same pressure and the resulting heat is recovered as low-pressure steam at 2-5 kg/cm.sup.2 G. The thus-produced steam is so much that a surplus of steam is still left even after it has been used in every steps of the urea synthesis system which steps are capable of utilizing such steam, for example, in the concentration step. On the other hand, a urea synthesis effluent which has been obtained by decomposing and separating unreacted substances with carbon dioxide is subjected to a low-pressure decomposition operation of 1-5 kg/cm.sup.2 G so as to decompose and remove substantially all the remaining unreacted substances. Furthermore, a urea synthesis effluent which has been obtained by decomposing and removing unreacted substances with ammonia still contains lots of ammonia therein. It is thus subjected to a medium-pressure decomposition operation of 10-25 kg/cm.sup.2 G, followed by subjecting the resulting stream to a low pressure decomposition operation of 1-5 kg/cm.sup.2 G. High to low pressure steam is used as a heat source for such medium-pressure and low-pressure decomposition operations. As a general rule, lots of low-pressure steam are recovered in the above-described stripping process but a large amount of high-pressure steam is on the other hand consumed in the stripping process, because, although the stripping operation in a stripping process becomes easier to practice as its pressure decreases, a urea synthesis melt obtained by conducting the synthesis of urea under low urea synthesis pressures has a poor urea synthesis yield and contains lots of unreacted substances and more high-pressure steam is required for the decomposition and separation of such unreacted substances.
In order to avoid such a difficulty as described above, it becomes necessary to conduct the synthesis of urea using ammonia in a still higher excess proportion and raising the urea synthesis pressure and temperature further. However, use of a higher urea synthesis pressure creates another difficulty, because the stripping operation has to be carried out at a higher pressure. Of course, it is possible to conduct the stripping operation at higher temperatures so as to facilitate the stripping operation. However, high-temperature stripping results in a drawback that more of the resulting urea undergoes hydrolysis.